1. Field of the Invention
This invention concerns a scanning optical system which forms a pattern on a scanning surface by scanning the surface with a laser beam.
2. Description of the Prior Art
In conventional scanning systems of this type, a laser beam, modulated by an ON-OFF modulator, is made to impinge on a polygon mirror, and a light beam reflected and deflected by the polygon mirror is converged onto a scanning surface by an f.theta. lens. As a result of scanning the beam on the scanning surface, a pattern is formed on the scanning surface.
In conventional scanning optical systems, to correct the misalignment of scanning lines due to tilt errors of the reflecting surface of the polygon mirror, the light beam may be brought to an image in an auxiliary scanning direction by for example combining a cylindrical lens and a toric f.theta. lens (Japanese Laid-Open Publication No. Sho 48-98844).
In this specification, the direction in which the laser beam is made to scan due to the polygon mirror will be referred to as the principal scanning direction, and the direction perpendicular to this direction will be referred to as the auxiliary direction.
As the light beam incident on the polygon mirror is generally inclined with respect to the optical axis of the f.theta. lens at an angle of 50.degree.-90.degree. , the deflection point (i.e., the point of intersection between the center axis of the incident beam and the reflecting beam on the reflecting surface) varies asymmetrically on both sides of the optical axis of the f.theta. lens due to rotation of the polygon mirror. The light beam incident on the polygon mirror is parallel to the principal scanning direction, but is either convergent or divergent in the scanning direction. The imaging point therefore moves in the auxiliary scanning direction due to the variation of deflection point, and the curvature of field on the scanning surface is asymmetrical in the auxiliary scanning direction.
If the curvature of field with respect to the optical axis is generated asymmetrically, and especially if it is generated as an odd function, the valve of the curvature increases in the positive sense even if it is decreased in the negative sense and vice versa, so it cannot be decreased on average. In correcting the curvature of field, therefore, rather than trying for example to make the curvature of field on the optical axis equal to 0, it is more important to limit the curvature of field over the whole scanning region by reducing its peak value such that it remains within the focusing depth.
In Japanese Laid-Open Publication No. Hei 2-23313, an apparatus is proposed wherein the curvature of field is generated symmetrically with respect to the optical axis by incorporating a lens in the scanning lens group which varies the radius of curvature in the auxiliary scanning direction, asymmetrically with respect to the optical axis. This asymmetrical lens however is difficult to manufacture, and it increases the overall cost of the apparatus.